1. Technical Field
The invention relates to continuous sine-wave commutation for brushless motor drivers and particularly to a method and circuit for start up of such motors where used in magnetic tape drives.
2. Description of the Related Art
A magnetic tape drive is a mechanism for controlling the movement of magnetic tape past a read or write head and for rewinding of the magnetic tape. A tape drive includes separate motors for driving a tape take-up reel and a tape supply reel. The motors are referred to here as the take-up motor and the supply motor, respectively.
In operating the take-up motor or tape supply motor it is important that tension be kept on the magnetic tape. The motors should not be operated in a fashion which results in the tape being pushed, or tension being released, for too long. If tape is pushed it can come off of its reel or leave the tape path with the possible destruction of the tape and the data thereon. Human intervention may be required to rewind or to replace the tape. Tape drive operation maintains tape linear velocity under tight control. Reel motor angular velocity varies with tape radius on the reels, which is continuously calculated by a motion control processor. Under these conditions tape tension is very difficult to control, which is why the collective and differential torques of the supply and takeup reel motors are directly controlled by the motion control processor.
As data density used with magnetic tape is increased, ever greater smoothness in take-up and supply motor torque is required. Commutation schemes to achieve smooth and highly controlled torque have relied on expensive brush commutated motors, or, in brushless motor designs, six-step commutation via Hall sensors. Six-step commutation causes, at best, a 14% variation in rotor torque at the commutation crossover points because motor torque versus shaft angle looks like a full-wave rectified three phase sine wave. A 12-step commutation system has been proposed which is claimed to achieve a 4% crossover dip, but is not known to have been applied to tape drives.
Commutation control for a brushless motor depends upon a scheme for synchronizing magnetic fields with rotor magnets. Commutation control is done by tracking phase (i.e. rotational) position of the motor shaft which carries the rotor magnet. Phase position of the shaft of a motor may be determined by use of an absolute position encoder. Absolute position encoders are, however, quite expensive. It would be cheaper to substitute an incremental encoder system which tracks change in position from a known starting point.
Certain problems arise from using an incremental encoder. For example, upon power up from a power interruption or shut down, the drive mechanism starts with no information about shaft phase. Until an index for the incremental encoder is found no shaft phase information will be available. As described in the article Design and Experimental Results of a Brushless AC Servo Drive, Pfaff et al., IEEE Transactions on Industry Applications, Vol. IA-20, No. 4 (July/August 1984), restart of a brushless AC servo motor after loss of phase information can be jerky and possibly in the wrong direction. To solve these problems, Pfaff proposed a startup controller with a limited output current and a direction detection element for reversing motor direction, if needed.
The need to protect a tape mounted on the tape drive mechanism from damage during startup, while searching for the shaft index mark, still further complicates use of synchronous AC servos with magnetic tape units and has prevented commercial application of incremental encoding to control commutation for brushless motors in automated tape drive mechanisms.